1. Field of the Invention
The present invention relates to a coil spring modeling apparatus capable of producing a reactive force (repulsive force) corresponding to compression of a helical spring such as a suspension coil spring, and a method of the same.
2. Description of the Related Art
As an example of a vehicle suspension system, a McPherson-strut-type suspension is known. The McPherson-strut-type suspension comprises a coil spring, and a strut (a shock absorber) provided inside of the coil spring. The coil spring is compressed by a load applied from above the coil spring, and is extended and retracted in accordance with the load. The strut is also extended and retracted.
In the McPherson-strut-type suspension, in order to reduce the sliding resistance of a strut, offsetting a force line position (FLP) of a coil spring from the center line of the coil spring is known. For example, the force line position (FLP) of a coil spring is set at a position where the friction of the strut is minimal. For this reason, the relationship between a force line position (FLP) of a coil spring and the sliding resistance of a strut must be specified. However, producing a variety of coil springs whose force line positions are different by way of trial is time consuming and costly. Thus, instead of using the coil spring, using a coil spring modeling apparatus has been proposed.
For example, a coil spring modeling apparatus disclosed in U.S. Pat. No. 7,606,690 (Document 1) is known. Also, an improved coil spring modeling apparatus is disclosed in “Research of Effect of Coil Spring Reaction Force Line on Vehicle Characteristics by Universal Spring” (Document 2), on pages 21 to 24 of the proceedings, presentation of which was made in the conference held by the Japan Society of Spring Engineers (in Nagoya) on Nov. 1, 2013, and “Experimental Study on the Effect of Coil Spring Reaction Force Vector on Suspension Characteristics” of SAE 2014 (Document 3), presentation of which was made in the U.S. (Detroit) on Apr. 8, 2014. The coil spring modeling apparatus disclosed in the above documents has a Stewart-platform-type parallel mechanism comprising six hydraulic cylinders. By actuating each of the hydraulic cylinders by fluid pressure, a reactive force corresponding to compression of a coil spring can be produced.
A coil spring has its length (deflection) changed in accordance with a compression load applied in the axial direction. The length (deflection) of a coil spring whose spring constant is invariable is increased in proportion to the load applied to the spring. Depending on the specification of a suspension comprising a coil spring, the characteristics (for example, the sliding resistance of a strut and a kingpin moment) may be changed in accordance with the change in length (the amount of compression) of the coil spring. Moreover, the spring constant is not necessarily invariable depending on the type of coil spring. For example, there exists a coil spring whose spring constant increases as the compression increases.
In the conventional coil spring modeling apparatuses disclosed in Documents 1 to 3, fluid having a constant pressure is always supplied to each of hydraulic cylinders in response to a load applied from above. Accordingly, each of the hydraulic cylinders always produces a constant reactive force in response to a load of compression applied from above. For this reason, a reactive force corresponding to the amount of compression cannot be produced in each of the hydraulic cylinders with the conventional coil spring modeling apparatus, and there is room for improvement to deal with the situation that the type of coil spring to be modeled is restricted.